Traveling-wave tube having a comb delay line formed on a ridge in a first waveguideand a plurality of connecting ridge waveguides



Nov. 14, 1967 YosHlo KATO ET AL 3,353,057

TRAVELING-WAVE TUBEHAVING A COMB DELAY LINE FORMED ON A RIDGE IN A FIRSTWAVEGUIDE AND A PLURALITY OF CQNNECTING RIDGE WAVEGUIDES Filed July 8,1964 4 Sheets-Sheet 1' jw/enlors yasln'o Kai-o Osgmu Konosu ATTORNEYSNov. 14, 1967 3,353,057

TRAVELING-WAVE TUBE HAVING A COMB DELAY LINE FORMED ON A YOSHIO KATO ETRIDGE IN A FIRST WAVEGUIDE AND A PLURALITY 0F CONNECTING RIDGEWAVEGUIDES Filed July 8, 1964 4 Sheets-Sheet 2 /2 F real/60% G c /0Frequency (60) H/ gs $35 F 7 53 Q ERQEEE I 05 FYI 140N051! 8/ M ,%%MM

ATTORNEYS Nov. 14, 1967 YOSHIO KATO ET AL 3,353,057

TRAVELING-WAVE TUBE HAVING A COMB DELAY LINE FORMED ON A RIDGE IN AFIRST WAVEGUIDE AND A PLURALITY 0F CONNECTING RIDGE WAVEGUIDES FiledJuly 8, 1964 4 Sheets-Sheet 5 Voltage svanmhg waverafio N G E Ina/enorsyoszio kaio (Q5 a mu kflnosq ATTORNEYS Nov. 14, 1967 YOSHIO KATO ET AL3,353,057

' TRAVELING-WAVE TUBE HAVINGA COMB DELAY LINE FORMED ON A RIDGE IN AFIRST WAVEGUIDE AND A PLURALITY 0F CONNECTING RIDGE WAVEGUIDES FiledJuly 8, 1964 4 Sheets-Sheefi 4 l o/fage sfand/ng Wave lOf/O Im/enz orsS/7io ka o Osamq Harms EXMMMM ATTORNE Y5 United States Patent Ofiice3,353,657 Patented Nov. 14, 1-967 ABSTRACT OF THE DIStILOSURE Atraveling wave type electron tube, for extremely high frequencies,provided with a comb-type slow wave circuit. The electron tube includesa wave energy absorbing attenuator provided at one end of the slow wavecircuit on the collector electrode side. The comb-type circuit isarranged so that the depth of the comb teeth gradually decreases at theend towards the electron gun and it is coupled to a ridge waveguidecontiguous therewith and composed of a ridge, which has the sametransverse width as the comb-type circuit, directed towards the bottomof 2 the comb so that an electromagnetic wave is introduced into afold-back waveguide from the fore end of the ridge waveguide to thecollector electrode side. Furthermore, this electron tube includes anairtight connection window provided at the output end thereof. Such anarrangement makes the'device compact and can eliminate reflection ofwaves in the discontinuous portion of the waveguide and thus provide agood matching characteristic with a constant output of a wide frequencyrange.

This invention relates to extremely high frequency electron tubes, andhas for its object to provide tubes particularly adapted for industrialmass production.

There have been proposed and devised various types of electron tubes foramplifying or generating extremely high frequency signals. Among themare electron tubes for treating signals of a few-millimeters inWavelength which are considered useful, if they utilize the interactionbetween an electromagnetic wave on a comb type circuit and an electronbeam.

In general, a comb type circuit is known as a slow wave circuit of sucha construction that a number of lateral slots are cut into the ridge ofa ridge waveguide, with a constant period, and the ridge is grooved orperforated longitudinally for passing an electron beam therethrough. Ina comb type backward Wave tube or a comb type traveling wave tubeemploying a comb type circuit, the tube is usually of slender and longconstruction in view of the arrangement of the electron-beam focusingapparatus, the electron beam being passed centrally in the axialdirection, and the input and output transmission lines are formed by useof rectangular waveguides.

However, any of the extremely high frequency electron tubes heretobeforeproposed with utilization of comb type circuits has not been of aconstruction adapted for industrial mass production nor of a convenientshape for practical use. Thus, for example, a comb type circuit is aslow wave circuit surrounded by a metal wall and an extremely highgeometrical correctness is required with respect to the central axis ofthe electron-beam groove or hole. Consequently, in the electron gun sideof such an electron tube, a special construction and dimensionalprecision are required for the couplings between input and outputwaveguides and the comb type circuit. For satisfying the 70 aboverequirements in old electron tubes employing comb type circuits,troublesome and time-consuming mechanical devices have been required forcutting out from a single metal block an integral body composed of thecomb portion and surrounding walls of complicated configurations.

Conventional input and output couplings heretobefore used arerectangular waveguide systems in which the waveguide openings aredisposed in a broad wall face in opposition to the comb teeth, namelythe upper wall of the comb type circuit at the opposite ends thereof andthe axes of the input and output waveguides at the openings areperpendicular to the electron beam axis. In the case when the windows ofthe input and output waveguides are disposed at the side of thecollector electrode, the waveguides are bent in the respectivedirections between coupling portions and the tube envelope by 90 degreesand directed to the collector electrode side. In such a conventionalconstruction, however, mode conversion of the electromagnetic wave iseffected abruptly at the coupling portion from the comb type circuit tothe Waveguide, and there being a resonance circuit included in thecoupling portion in many cases, good matching characteristics cannot beobtained in a broad frequency band. In addition, the comb portion andthe rectangular waveguide portion have been manufactured separately. Ithas been difficult to keep precision in assembly, resulting in a narrowfrequency band in which good matching is possible. Further, the coreportion of the electron tube is of peculiar form and large size so thatit is not suitable for practical use.

The present invention provides an extremely high frequency (EHF)electron tube adapted for practical use, that can solve the variousproblems described above.

An additional object of the present invention is to provide an EHFelectron tube having a small reflection and a small loss over a broadfrequency band.

Another object of the invention is to provide an electron tube havinggood characteristics over a broad frequency band, with a high yield ratein industrial production and adaptability for mass production withrespect to construction.

A further object of the invention is to provide an EHF electron tube ofshort dimension, yet with good characteristics over a broad frequencyhand.

There are other objects and particularities of the present invention,which will be made obvious from the following detailed description, withreference to the accompanying drawings, in which:

FIGS. 1 and 2 show a backward wave oscillator tube embodying the presentinvention, in cross section and longitudinal section, respectively;

FIG. 3 shows a process of manufacturing the combined comb teeth andridge in the embodiment shown in FIGS.

1 and 2;

FIG. 4 is a detailed perspective sectional view of a portion of FIG. 2,with representation of various dimensions;

FIGS. 5 and 6 are graphs showing performances of the electron tubesshown in FIGS. 1 to 4;

FIG. 7 shows the detailed construction of a part shown in FIG. 2;

FIG. 8 is a sectional view taken at line A-A in FIG. 7; I

FIGS. 9 and 10 are graphs showing performances of the electron tube ofthe invention;

FIG. 11 shows a modified form of the tube shown in FIGS. 1 and 2;

FIG. 12 is a longitudinal sectional view of a traveling wave amplifiertube embodying the invention; and

FIG. 13 is a cross-sectional view of another embodiment of theinvention.

Referring now to FIGS. 1 and 2, the backward wave tube illustratedcomprises a metal strip 1 of a thickness equalto the lateral width ofcomb teeth k, a and b formed on the upper longitudinal side of the stripalong the effective length of the same. At the end portion of the stripnear the electron gun, between the teeth b and corner 0, several lateralslots gradually diminishing towards the corner are provided in order tolessen reflection of the electromagnetic .wave, while at the other endportion near to the collector electrode 10, that is near to the teeth k,

the teeth have gradually decreasing height in order to lessen theelectromagnetic wave reflection. At the side near to the collectorelectrode 10, on the lower longitudinal side of the strip 1, there isformed a tapered portion at for converting a ridge waveguide to arectangular by brazing or otherwise fastening metal members 2 and 3 tothe metal strip 1 with precise positional relations. The upper wall ofthe comb type circuit is formed by a metal member 4. The composite partconsisting of members 1, 2 and 3 is enclosed in a metal member of achannel-shape cross section, members 4 and 5 being fixed :together by asuitable number of set screws.

A pair of opposite supporter metal plates 6 and 7 are provided with acentral electron-beam slit 14 and a cen- 'tral port 15 for anattenuator, respectively, and have peripheral shapes fitting the insidewall of the cylindrical envelope 8 of the tube.

The combination of the above-described members 1 through 7 constitutes astructure having a comb type circuit k, a, b at the upper portion, aridge waveguide c-d at the side portion, and a ridge waveguide d-e and ataperridge waveguide e-f at lower portion, and is inserted in thecylindrical envelope'8. 'For'taking out the output, a

rectangular waveguide 9 is provided passing through the collectorelectrode 10- and having a passage-h in continuation with a rectangularport g of the metal plate 7 and the taper-ridge waveguide e.-f. Thecollector electrode 10 is provided with a collector hole 11. In theopposite end-portion of envelope 8, an electron gun 12 is mounted forgeneration of an electron beam 13.

The electron beam 12 interactswiththehigh-frequency electric field onthe comb type circuit, under an appropri' ate accelerating voltage inthe well-known manner, and

the generated electromagnetic wave is transmitted by Way rectangularwaveguide outside-the electron tu'be. Here,

the portion [2-0 is the conversion portion from the comb type circuit tothe ridgewaveguide, the portion c-a' is the vertical ridge waveguide,the portion d-e'is the fold-back ridge wavegnide,-the portion e-f is theconversion portion from the ridge waveguide to the rectangularwaveguide,the portion g-h is the rectangular waveguide, and the portions0 and d are corners of the ridge waveguides.

'The angle of bent corner 0 is 910*degrees, while that of corner d may'be a right angle or an obtuse angle. In-

stead, there'may be employed a bent ridge waveguide at the corner d.

*According to the invention, the ridge of the ridge waveguide, directedto the bottom of the slots of the comb type circuit at-the electron-gunside of the comb type circuit, and the ridge of the fold-back waveguide,as well as the comb teeth, are formed as one body from a single metalstrip, whereby reflection of the electromagnetic -.wave is negligiblysmall, and good matching characteristics are obtained with a constantoutput over a wide frequency band. Further, by use of-the single strip,as above-described with one longitudinal side provided with comb teethand one lateral side and the other longitudinal side forming ridges, thepassage of an electro-magnetic wave can be bent into a reclined-U-shape,so that the whole structure may be accommodated within a cylindricalenvelope in a compact form of extremely practical usage.

Referring to FIG. 3, eight metal strips 1 are fixed as one body inlateral alignment between a holding wall 21 of a mount base 20 and aholding plate 22. By rotating the cutter 23 mounted on a shaft 24supported in a bearing 25 being mechanically reciprocated in thedirections of arrows shown the lateral slots 26 may be provided in themetal strips 1. According to the invention, a number of metal strips 1may be subjected to mechanical working at the same time, asabove-mentioned, obviating the relatively long period of time requiredby ordinary individual machining. Consequently, precise and efficientcutting is possible by the above-mentioned simultaneous machining.

The connection between the waveguide 9 and the external waveguide 30shown in the right-hand portion of FIG. 2 is now to be explained. Thisconnection is designed substantially for the elimination of reflectionand loss of the electromagnetic wave over the whole frequency band usedin the rectangular waveguide, while assuring easy sealing between thedielectric disk and the metal parts in the hermetic window constructionof the waveguide for an electron tube of themillimeter wave region.

Referring to FIGS. 2 and 4, a thin cylindrical-space 31 is formed in anend flange 35 of the rectangular waveguide '9, and has a thickness Wwhich is small in comparison to its diameter D The flange 35is made ofiron-nickel-cobalt alloy, or copper,-etc., and is provided with anannular angular cut 34 around the cylindrical-space 31 in the butt face32 of flange 35, for receiving a dielectric disk 33 of diameter D, at aright angle to the axis of waveguide 9, in a sealing condition. Thedielectric disk 33 is made of glass, ceramics, sapphire, quartz orthelike, and is posi-.

tively sealed to the flange 35 in the annular angular cut 34 by means ofglazing, brazing, or other appropriate -means depending upon thedielectric material used. The

external waveguide 30, in axial alignment with the waveguide 9, has anend flange 36 having a thin cylindrical space 37 of the same dimensionsas the space 31 and in register with the latter. By sucha construction,the dielectric disk 33 can be positively sealed to the flange 35 alongthe peripheral face having width t, as well as, the annular edge portionof the disk plane having a radial width of A: (D D There might occurcertain disturbance of the electromagnetic field by virtue of theexistence of the annular angular cut 34 of /2(D -D radial width, butsuch a disturbance does not affect the transmission characteristicsdisadvantageously, if the thickness W and diameter D of the spaces 31and 37 are selected in an appropriate dimensional relation, and goodtransmission characteristics can be obtained over a broad frequencyband.

Thus, referring to FIG. 4, it has been found that if the thickness t andspecific dielectric constant e of the dielectric disk 33 and theinternal lateral widths a of waveguides 9 :and30 are selected inthe'range of 0.15 oc ext 0.5ot,

and thediameter D and thickness W of the spaces 31 and 37, and thediameter D, of the dielectric disk 33 are selected in the ranges of 0.7uD l.0ot, 0.05oc W 0.250t, and 0.1a (D -D O.3a, then the electric fieldcomponent of the transmittedelectromagnetic wave .is distributed tobecome substantially perpendicular to the plane :of dielectric disk 33,and the voltage standing-wave ratio (VSWR) and the transmission loss aresufliciently de creased over the whole'frequency band used in theme-'tangular waveguides 9 and 30.

ly, in comparison to a conventional hermetic window inserted in achoke-flange connection, positive and good hermetic sealing can beaccomplished. Tests show that a superior voltage standing wave ratio andtransmission loss are obtained as shown in FIGS. 5 and 6 with the newconnecting construction. In these tests, the connecting construction hadthe following dimensions:

a=22.9 millimeters, Do=O-960t, D1'D0=0-1750t, W=0.196a,

As seen from these figures of drawings, within the frequency range from8.2 gigacycles to 12.4 gigacycles, the voltage standing wave ratio islower than 1.14 (FIG. 5) and the transmission loss is lower than 0.4decibel (FIG. 6). Such extremely superior characteristics can beobtained in any region of extremely high frequency by virtue of the lawof similarity of electromagnetic waves. The new connecting constructionis extremely simple and easy to manufacture and is adapted forindustrial mass production.

The construction of the central port 15 for the attenuator in thesupporter metal plate 7 will now be described in detail with referenceto FIGS. 7 and 8.

The comb teeth of the comb type circuit decrease in height graduallynear to k at the collector-electrode end of teeth a, and at the end ofmetal member 4 the circuit is converted to a rectangular waveguide 40.The supporter metal plate 7 between the waveguide 40 and the collectorhole 11 is provided with a short cylindrical port 15 passingtherethrough and having a diameter D,, larger than the lateral width orof rectangular waveguide 40, the axis of port 15 being in coincidencewith the axis of waveguide 40. Within the cylindrical port 15, one ormore dielectric plates 42, of small thickness T in comparison tothewavelength of the electromagnetic wave under consideration andcovered by films 41 of resistance material, are arranged in parallel tothe narrower wall of the rectangular waveguide 40, the edges of theplates 42 being fixed to the plate 7 in grooves 43 formed therein byglass-sealing, brazing, or simple insertion. The collector hole 11formed in the collector electrode is of a pore form of a diameter Esufiicient to contain the cross-sectional area of electron beam 13 sothat feedback of the secondary electron may be prevented.

With the above-mentioned construction, the forward progressive wavedirected to the collector electrode, which is a component of theelectro-magnetic wave generated in the comb type circuit that is notnecessary for the backward wave oscillator tube, is after introductioninto the cylindrical port 15, divided into two components that proceedin the upward and downward directions, respectively, as shown by chainlines 44 in FIG. 7, their electric field components being distributed asshown by arrowed lines 45 so that the resistance films 41 acteffectively to damp the energy of the forward progressive wave. FIG. 9shows the relation between frequency and the voltage standing wave ratioas measured from the side of the rectangular waveguide 40. In themeasurements, two dielectric plates 42 were employed with the followingconstructional dimensions:

8 0.45m, Da 1.8a, Ca 0.870s, Xa 0.75m, Ta 0.09m,

dimensions ,8, Da, Ca, Xa and Ta being as illustrated in FIGS. 7 and 8and the specific dielectric constant e 5. In FIG. 9, the absissa showsthe frequency normallized by the lower cutoff frequency fl of therectangular wave guide, that is, f/fl and their opposite endssubstantially correspond to the normal range of use of the rectangularwaveguide.

As is clearly seen from FIG. 9, in the attenuator of the aboveconstruction the voltage standing wave ratio is below 1.1 over thesubstantial portion of the frequency band used in the rectangularwaveguide, and it is suitable as the terminating attenuator for a slowwave circuit of a backward wave oscillator tube by reason of its broadoperating frequency range.

This attenuator can also be of short dimension in the longitudinaldirection of a slow wave circuit, is easy to manufacture by reason ofits simple construction, and consequently, is very useful from theindustrial view point as an attenuator for the termination of a slowwave circuit.

In FIG. 10 there is shown the whole performance, as measured at an outerflange of the waveguide 30, of the extremely high-frequency backwardwave oscillator as shown in FIGS. 1 and 2. In the construction showntherein, wall-to-teeth spacing l and wall-to-ridge spacing l 1 are allequal, and l =l =l 0.2 millimeter. The heights of the comb type circuitin the direction of A- and ridge waveguide are 1.2 millimeters and 1.55millimeters, respectively; the lateral widths of the upper wall of thecomb type circuit and ridge waveguides are 3.1 mm., respectively; thelength of the shorter side of metal strip 1 is m=8 millimeters; and thelateral widths of comb teeth and ridges are n=1.05 millimeters. Thefrequency band ranges from 55 go. to 77 gc. In FIG. 10, the abscissashows j/f the frequency normallized by the upper cutoff frequency of thecomb type circuit f =82 gc., While the ordinate shows voltage standingwave ratio (VSWR). As is clear from FIG. 10, the VSWR of the presentembodiment of the invention is sufiiciently low and satisfactory as abackward wave tube.

In the modification shown in FIG. 11, a short ridge waveguide d-d isinserted between the vertical ridge waveguide cd, perpendicular to theelectron beam axis at the end near to the electron gun, and thefold-back ridge waveguide d'e'f and the waveguide d-d' has aninclination intermediate both waveguides cd and de-f. In addition, thefold-back waveguide d'e'f is a long and gentle taper-ridge waveguide andis converted to a rec tangular waveguide at the output end 1. Byutilization of the slanted ridge waveguide d-d and the long and gentletaper ridge waveguide d-e-f the transmission characteristics have beengreatly improved.

In FIG. 12 there is shown a traveling wave amplifier tube embodying theinvention. The construction of this embodiment is similar in general tothe backward wave oscillator tube shown in FIGS. 1 and 2, and thedescription will be given below only on the parts different from thelatter. Contrary to the case of FIG. 2, the input electromagnetic waveis introduced through a rectangular waveguide 9 and proceeds by way of hgfe d c b a while, under interaction with the electron beam, the wave isamplified and the amplified output is transmitted by way of k qr s to anoutput waveguide 16 from which the output is taken.

It will readily be understood that a traveling wave amplifier and abackward wave amplifier may be of one and the same construction,provided that the operational conditions are converted from each other,the only difference therebetween being that the directions of input andoutput are reversed from each other. Thus, a backward wave amplifiertube can be obtained by use of the construction shown in FIG. 12.

FIG. 13 shows a still further embodiment of the invention, in which thecomb-teeth portion of metal strip 1 is provided with a through hole 102for transmission of an electron beam, intsead of a longitudinal groove103 in the embodiment shown in FIGS. 1 and 2. The operation of thepresent embodiment is substantially the same with that of FIGS. 1 and 2.

What we claim is:

1. An extremely high frequency electron tube .of the traveling wave typecomprising a substantially cylindrical enclosure having at one endthereof anrelectron gun and atthe opposite end thereof a collectorelectrode arranged in alignment with said electron gun, a comb-typecircuit formed on a ridge in a first ridge waveguide, said ridge of saidfirst ridge waveguide being a metal strip having a plurality oftransverse slots therein thereby forming comb teeth, said comb-typecircuit being mounted in said cylinder between said electron gun andsaid collector electrode along the path of travel of the electron beamfrom said gun, said comb teeth being gradually diminished in height atthe end-of said circuit towards said electron gun, a second ridgewaveguide transversely mounted in said cylinder directed towards thebottom of said comb teeth and being continuous therewith, said secondridge having the same transverse width as said comb teeth, and afold-back waveguide directed towards the collector electrode andconnected to the fore end of said second ridge waveguide, said fold-backwaveguide including a tapered portion.

2. The electron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridge Waveguide.

3. The electron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridge waveguide and isconstituted by butting two metal members, uponboth sides ofsaid metalstrip having comb teeth, said members having faces defining the bottomwall of said first ridge waveguide and forming the ridge after assembly.

4. The electron tube according to claim 1, in which a shortcylindricalport surroundedby a conductor is connected to a rectangularwaveguide at the collector electrode side of said comb type circuit,said port having a diameter larger than the-lateral width of saidrectangular waveguide and an axis incoincidence with theaxis of saidrectangular waveguide, at least one dielectric plate mounted ,withinsaid cylindrical sport, said plate being covered with films ofresistance material attached on its surfaces and having a smallthickness in comparison to the wavelength of theelectromagnetic wave ofsaid rectangular waveguide,-and being in parallel withthe shorter wallof said rectangular waveguide,thusconstituting a backw-ardwaveoscillator.

5. The electron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having aridge incontinuation with the ridge of the second ridgewaveguide and in ,whichashort cylindrical port surrounded-by a conductor is connected to arectangular waveguide atthe collectorelectrode side ofsaid comb typecircuit, said port having a diameter 7 larger than the lateral width ofsaid rectangular waveguide and an axis in coincidence-withthe axis ofsaid rectangular waveguide, at least one dielectric plat mounted in saidcylindrical port, said platebeing covered withtfilms of resistancematerial attached on itssurfaces and having a small thickness incomparison to the wavelength of the electromagnetic wave ofsaidrectangular waveguide, and being in parallel with the shorter ,wall ;ofsaid rectangular waveguide, thus constitutinga backward wave oscillator.

,6. The electron tube according to claim 1 in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation withthe ridge of the second ridge waveguide and isconstituted by butting two-metal members upon both sides of the metalstrip having comb teeth at the essential-portion said metal membershaving faces defining the bottom Wall of said first ridge waveguide andforming theridge after assembly, a short cylindrical port surrounded bya conductor connected to a rectangular waveguide at the collec- ,torelectrode side of said comb type circuit, said port having a diameterlarger than the lateral width of said rectangular waveguide andtan axisin coincidence with covered with films of resistance material attachedon its surfaces and having a small thickness in comparison to thewavelength of the electromagnetic wave of said rectangular waveguide,and being in parallel with the shorter wall of said rectangularwaveguide, thus constituting a backward wave oscillator.

7. The electron tube according to claim 1, in which the output end ofsaid fold-back waveguide is converted to a rectangular waveguide, a thincylindrical space formed in an enclosure at the connecting portionbetween said rectangular waveguide and an external rectangularwaveguide, said enclosure being composed of two parts with butt facestherebetween, an annular angularcut being formed in the butt face of theelectron-tube side, and a dielectric disk being placed in said annularangular cut perpendicular to the axis of said rectangular waveguides andsealed to said annular angular cut.

8. The electron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridge waveguide and in whichthe output end of said fold-back waveguide is converted to a rectangularwaveguide, a thin cylindrical space formed in an enclosureat theconnecting portion between said rectangular waveguide and .an externalrectangular waveguide, said enclosure being composed of two parts withbutt faces therebetween, an annular angular cut being formed in the buttface of the, electron-tube side, and a dielectric disk being placed insaid annularangular cut perpendicular to the axis of said rectangularwaveguides and sealed to said annular angular cut.

9. The electron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridge waveguide andconstituted by butting two metal members upon both sides of the metalstrip having comb teeth at the essential portion, said metal membershaving faces defining the bottom wall of said first ridge waveguide andforming the ridge after assembly,

the output end of said fold-back waveguide being converted .to arectangular waveguide, a thin cylindrical space formed in an enclosureat the connecting portion between said rectangular waveguide and anexternal rectangular waveguide, said enclosure being composed of twoparts with butt faces therebetween, an annular angular out being formedin thebutt face of the-electron- .tube side, and a dielectric disk beingplaced in said annular angular cut perpendicular to the axis of saidrec:

tangular waveguides and sealed to said annular angular cut.

10. The electron tube according to claim 1, in which .the main portionof said fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridgewaveguide and constitutedby butting two metal members upon both sides of the metal strip havingthe comb teeth at the essential portion, said metal members having facesdefining the bottom wall of said first ridge waveguide and forming theridge after assembly, the output end of said fold-back waveguide beingconverted to a rectangular waveguide, a thin cylindrical space formed inan enclosure at the connecting portion between said rectangularwaveguide and anexternal rectangular waveguide, said enclosure beingcomposed of two parts with butt faces therebetween, an annular angularout being formed in the butt face of the electron-tube side, and adielectric disk being placed in said annular angular cut perpendicularto the axis of said rectangular waveguides andsealed to said annularangular cut.

11. Theelectron tube according to claim 1, in which the main portion ofsaid fold-back waveguide is a third ridge waveguide having a ridge incontinuation with the ridge of the second ridge waveguide, in which ashort cylindrical port surrounded by a conductor is connected to arectangular waveguide at the collector electrode side of said comb typecircuit, said port having a diameter larger than the lateral Width ofsaid rectangular waveguide and an axis in coincidence with the axis ofsaid rectangular waveguide, at least one dielectric plate mounted withinsaid cylindrical port and covered with films of resistance materialattached on its surfaces and having small thickness in comparison to thewavelength of the electromagnetic wave of said rectangular waveguide,and being in parallel with the shorter wall of said rectangularwaveguide, thus constituting a backward wave oscillator, the output endof said fold-back waveguide being converted to a rectangular waveguide,a thin cylindrical space formed in an enclosure at the connectingportion between said rectangular waveguide and an external rectangularwaveguide, said enclosure being composed of two parts with butt facestherebetween, an annular angular cut being formed in the butt face ofthe electrontube side, and a dielectric disk being placed in saidannular angular cut perpendicular to the aXis of said rectangularwaveguides and sealed to said annular angular cut.

References Cited HERMAN KARL SAALBACH, Primary Examiner. ELI LIEBERMAN,Examiner.

S. CHATMON, 111., Assistant Examiner.

1. AN EXTREMELY HIGH FREQUENCY ELECTRON TUBE OF THE TRAVELING WAVE TYPECOMPRISING A SUSBTANTIALLY CYLINDRICAL ENCLOSURE HAVING AT ONE ENDTHEREOF AN ELECTRON GUN AND AT THE OPPOSITE END THEREOF A COLLECTORELECTRODE ARRANGED IN ALIGNMENT WITH SAID ELECTRON GUN, A COMB-TYPECIRCUIT FORMED ON A RIDGE IN A FIRST RIDGE WAVEGUIDE, SAID RIDGE OF SAIDFIRST RIDGE WAVEGUIDE BEING A METAL STRIP HAVING A PLURALITY OFTRANSVERSE SLOTS THEREIN THEREBY FORMING COMB TEETH, SAID COMB-TYPECIRCUIT BEING MOUNTED IN SAID CYLINDER BETWEEN SAID ELECTRON GUN ANDSAID COLLECTOR ELECTRODE ALONG THE PATH OF TRAVEL OF THE ELECTRON BEAMFROM SAID GUN, SAID COMB TEETH BEING GRADUALLY DIMINISHED IN HEIGHT ATTHE END OF SAID CIRCUIT TOWARDS SAID ELECTRON GUN, A SECOND RIDGEWAVEGUIDE TRANSVERSELY MOUNTED IN SAID CYLINDER DIRECTED TOWARDS THEBOTTOM OF SAID COMB TEETH AND BEING CONTINUOUS THEREWITH, SAID SECONDRIDGE HAVING THE SAME TRANSVERSE WIDTH AS SAID COMB TEETH, AND AFOLD-BACK WAVEGUIDE DIRECTED TOWARDS THE COLLECTOR ELECTRODE ANDCONNECTED TO THE FORE END OF SAID SECOND RIDGE WAVEGUIDE, SAID FOLD-BACKWAVEGUIDE INCLUDING A TAPERED PORTION.